1. Field of the Invention
The present invention relates to an organic thin film light emitting device used in a flat light source, a flat display or the like. More particularly, the present invention relates to a light emitting device using an organic compound, and more specifically to a device improved in durability by employing a fluorine-containing organic compound in a light emitting portion.
2. Related Background Art
Conventional examples of organic light emitting devices include a device that emits light when a voltage is applied to a vapor-deposited anthracene film (Thin Solid Films, 94 (1982) 171). In recent years, however, because of the advantages that formation of a large light emitting area can more easily be attained in the organic light emitting device than the inorganic light emitting device, desired colors can be obtained by the development of various new materials, and the devices can be driven at a low voltage, application studies on the organic light emitting device for forming devices, including material development, have been made actively.
For example, as is described in detail in Micromol. Symp. 125, 1-48 (1997), an organic electroluminescence (EL) device, which is a representative example of the organic light emitting device, has generally a structure in which upper and lower electrodes, and an organic layer including a light emitting layer formed between the electrodes are provided on a transparent substrate. The basic constitution thereof is shown in FIGS. 1A and 1B.
As shown in FIGS. 1A and 1B, an organic EL device is generally composed of a transparent electrode 14 and a metal electrode 11 on a transparent substrate 15, and a plurality of organic layers disposed between the electrodes. Like features in the respective figures are indicated with like numerals.
In FIG. 1A, an organic layer consists of a light emitting layer 12 and a hole transport layer 13. As the material for the transparent electrode 14, a material with a large work function, such as ITO, is used to achieve good hole-injecting characteristics from the transparent electrode 14 to the hole transport layer 13. As the material for the metal electrode 11, a material with a small work function, such as aluminum, magnesium and alloys thereof, is used to achieve good electron-injecting characteristics to the organic layer. These electrodes generally have a thickness of 50-200 nm.
For the light emitting layer 12, an aluminoquinolinol complex (a representative example is Alq3 shown in the following chemical formulas I) or the like is used. As the hole transport layer 13, for example, an electron-donating material such as a biphenyldiamine derivative (a representative example is xcex1-NPD shown in the following chemical formulas I) is used.
The organic EL device constructed as described above has a rectifying property, and when an electric field is applied so as to make the metal electrode 11 act as a cathode and the transparent electrode 14 act as an anode, electrons are injected from the metal electrode 11 into the light emitting layer 12, and holes are injected from the transparent electrode 14 into the light emitting layer 12.
When the injected holes and electrons recombine in the light emitting layer 12, excitons are formed, and light is emitted in the process of the radiation and deactivation of these excitons. At this time, the hole transport layer 13 plays the role of an electron blocking layer to raise the recombination efficiency in the light emitting layer 12/hole transport layer 13 interface, and to enhance light emitting efficiency.
Furthermore, in FIG. 1B, an electron transport layer 16 is formed between the metal layer 11 and the light emitting layer 12 of FIG. 1A. Thus, by independently forming the electron transport layer 16 to isolate the light emitting function from the electron/hole transport function, and making more effective carrier blocking constitution, efficient light emitting can be performed. As the material for the electron transport layer 16, for example, an oxadiazole derivative or the like can be used.
Heretofore, in light emission generally used in the organic EL device, the excited state includes an excited singlet state and an excited triplet state. A light emission accompanying a transition from the former state to a ground state is referred to as fluorescence, while a light emission accompanying a transition from the latter state to a ground state is referred to as phosphorescence; and substances in these states are referred to as a singlet exciton and a triplet exciton, respectively.
Many of the organic light emitting devices so far studied utilize fluorescence generated in transition from the singlet exciton to a ground state. Recently, on the other hand, devices that utilize phosphorescence through triplet exciton have been studied.
The representative documents published concerning the results of such studies are: Document 1: D. F. O""brien et al., xe2x80x9cImproved energy transfer in Electrophosphorescent devicexe2x80x9d, Applied Physics Letters Vol. 74, No. 3, p. 422 (1999); and document 2: M. A. Baldo et al., xe2x80x9cVery high-efficiency green organic light-emitting devices based on Electrophosphorescencexe2x80x9d, Applied Physics Letters Vol. 75, No. 1, p. 4 (1999).
In these documents, as shown in FIG. 1C, an organic layer of a four (4) layer structure is mainly used. In this structure, a hole transport layer 13, a light emitting layer 12, an exciton-diffusion preventing layer 17 and an electron transport layer 16 are stacked in the named order from the anode side. The materials used therein are carrier transport materials and phosphorescent materials shown in the following chemical formulas I. The term xe2x80x9cphosphorescent materialxe2x80x9d used herein is intended to mean a material having phosphorescent properties at around 20xc2x0 C.
The abbreviations of materials in the following chemical formulas I stand for the following means:
Alq3: aluminoquinolinol complex;
xcex1-NPD: N4,N4xe2x80x2-di-naphthalen-1-yl-N4,N4xe2x80x2-diphenyl-biphenyl-4,4xe2x80x2-diamine;
CBP: 4,4xe2x80x2-N,Nxe2x80x2-dicarbazole-biphenyl;
BCP: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline;
Bphen: 4,7-diphenyl-1,10-phenanthroline;
PtOEP: platinum-octaethylporphyrine complex; and
Ir(ppy)3: Iridium-phenylpyridine complex.

Devices that showed high light-emitting efficiencies in both Documents 1 and 2 were constituted by using xcex1-NPD as the hole transport layer 13, Alq3 as the electron transport layer 16, and BCP as the exciton-diffusion preventing layer 17; and the light emitting layer 12 was constituted using CBP as the host material, and incorporating PtOEP or Ir(ppy) 3 as a phosphorescent material in a concentration of about 6% thereinto.
The reason why the phosphorescent material particularly attract attention is that a high light emitting efficiency can be theoretically expected. Specifically, excitons formed by the recombination of carriers consist of singlet excitons and triplet excitons, and the probability of the formation thereof is 1:3. Although conventional organic EL devices utilized fluorescence during the transition from the singlet exciton state to the ground state as light emission, the light emitting yield was theoretically 25% relative to the number of formed excitons, and this was the theoretical upper limit. However, if phosphorescence from excitons generated from the triplet state is used, at least 3-fold yield is theoretically expected, and when the transfer by the intersystem crossing from the singlet state of a high energy level is considered, 4-fold, that is 100%, light emitting efficiency can be theoretically expected.
International Publication No. WO 02/02714 discloses an example of a device using a fluorine-containing compound in a light emitting layer, and reports that use of a fluorinated material only as a guest material, which is a material that contributes to light emission, inhibited lowering of the light emitting efficiency even if the content of the guest material was elevated, compared with the case wherein a non-fluorinated material was used.
The light emission lifetime of an organic light emitting device is affected by factors, such as the glass transition temperature and the stability to electric charge of the material for the charge transport layer and the light emitting material, and the interfacial state between layers; and furthermore, in the host-guest light emitting layer wherein two or more components of conductive host materials and light emitting guest materials are mixed, by various factors, such as the dispersibility and the content of each organic material, stability during vapor deposition, and moisture content.
Thus, there are generally a large number of factors affecting the lifetime of an organic light emitting device. Therefore, there is a continuing need for a device constitution that enables the elongation of the lifetime of organic light emitting devices.
It is, therefore, an object of the present invention to provide an organic light emitting device that has a high light emitting efficiency and a long light emission lifetime.
According to the present invention, there is provided an organic light emitting device comprising a pair of electrodes provided on a substrate, and a light emitting layer comprising at least one layer of an organic substance provided between the electrodes, wherein the light emitting layer comprises a first organic substance and a second organic substance mainly concerning a light emission wavelength of the light emitting layer, and wherein the first organic substance comprises a fluorine-containing organic compound.